Late afternoon lighting produced a dramatic shadow of NASA's Mars Exploration Rover Opportunity photographed by the rover's rear hazard-avoidance camera on March 20, 2014.

The shadow falls across a slope called the McClure-Beverlin Escarpment on the western rim of Endeavour Crater, where Opportunity is investigating rock layers for evidence about ancient environments. The scene includes a glimpse into the distance across the 14-mile-wide (22-kilometer-wide) crater.

The rover experienced a partial cleaning of dust from its solar panels by Martian wind this week, boosting electrical output from the array by about 10 percent, following a similar event last week. That is in addition to increased sunshine each day in the Martian southern hemisphere's early spring. Combined, the seasonal effect and multiple dust-cleaning events have increased the amount of energy available each day from the rover's solar array by more than 70 percent compared with two months ago, to more than 615 watt hours.

On March 23, 2004, when Opportunity had been working on Mars for only two months, scientists announced the mission's headline findings of evidence for water gently flowing across the surface of an area of Mars billions of years ago.

During Opportunity's first decade on Mars and the 2004-2010 career of its twin, Spirit, NASA's Mars Exploration Rover Project yielded a range of findings proving wet environmental conditions on ancient Mars -- some very acidic, others milder and more conducive to supporting life.

Harvard University researchers Fabien Paulot and Daniel Jacob used computer models including a NASA model of chemical reactions in the atmosphere to better represent how ammonia interacts in the atmosphere to form harmful particulate matter. The improved simulation helped the scientists narrow in on the estimated health costs from air pollution associated with food produced for export – a growing sector of agriculture and a source of trade surplus.

"The 'cost' is an economic concept to measure how much people are willing to pay to avoid a risk," Paulot said. "This is used to quantify the cost for society but also to evaluate the benefits of mitigation."

The new research by Paulot and Jacob calculate the health cost associated with the ammonia emissions from agriculture exports to be $36 billion a year – equal to about half of the revenue generated by those same exports – or $100 per kilogram of ammonia. The study was published December 2013 in Environmental Science & Technology.

The new estimate is about double the current estimate by the U.S. Environmental Protection Agency, which suggests a cost of $47 per kilogram of ammonia. The scientists say the new estimate is on the high end of the spectrum, which reflects the need for more research into characterizing the relationship between agricultural ammonia emissions and the formation of the harmful fine particulate matter – a relationship that's not as straightforward as previous estimates assumed.

"The effect of ammonia on fine particulate is complex, and we believe that the models previously used in the United States to price ammonia emissions have not captured this well," Paulot said.